A lithium secondary battery that is a nonaqueous electrolyte battery is a device having a high energy density and a high power density. Heretofore, lithium transition metal composite oxides such as LiCoO2, LiNiO2 and LiMn2O4 have been put into practical use as positive electrode materials for lithium secondary batteries. These lithium transition metal composite oxides have a capacity of about 150 mAhg−1 per mass.
Sulfur has a high theoretical capacity of 1675 mAhg−1 per mass. Sulfur has been examined as a positive electrode material exhibiting a capacity that is 10 times or more of the capacity of a conventional material. However, sulfur has not been put into practical use yet because it has high reactivity with an electrolyte solution, and high resistance.
Attempts have been made heretofore to combine sulfur with an electrically conductive material.
Non-Patent Document 1 describes that sulfur is mixed with porous carbon into which lithium ions can be inserted, and the mixture is heat-treated at 400° C. for 6 hours to prepare an electrode material composed of a sulfur-carbon composite in which sulfur is carried on porous carbon. Non-Patent Document 1 also describes a lithium-sulfur secondary battery obtained using the electrode material. According to this document, the capacity of sulfur on a mass basis is 720 mAhg−1 after 100 cycles of charge-discharge.
Patent Document 1 describes that sulfur is mixed with a porous carbon matrix having nanopores and nanochannels with a mean diameter of 1 to 999 nm and having a pore volume of 10 to 99% by volume, and the mixture is heat-treated at 120 to 180° C. to prepare a porous matrix in which sulfur is adsorbed to a part of the nanopores and nanochannels. According to this document, sulfur constitutes 70% by mass of the material.
Patent Document 2 describes a mesoporous carbon composite material to be used in an electrode of a nonaqueous electrolyte-type secondary battery, the composite material containing sulfur in an amount of 5% or more of the total weight in mesopores of mesoporous carbon with a pore mean diameter of 1 to 40 nm.